The present invention relates to a loopback cell control system for measuring the time spent to perform transmission through forward paths by using a plurality of ATM (Asynchronous Transfer Mode) apparatuses and the time spent to perform transmission through backward paths in a loopback test in an ATM switching network and, more particularly, to a loopback cell control system which can monitor a delay time in each transmission path.
To test the normalities of a plurality of ATM apparatuses and transmission paths in a conventional ATM switching network, various loopback cell control systems have been proposed, which are designed to transfer a test loopback cell through an arbitrary route and check the validity of the loopback cell after it is circulated once.
FIG. 5 shows a conventional loopback cell control system. Referring to FIG. 5, three ATM apparatuses 41a, 41b, and 41c are cascaded through two forward paths 42a and 42b and two backward paths 43a and 43b to constitute an ATM switching network. The ATM apparatuses 41a, 41b, and 41c respectively include time managing sections 44a, 44b, and 44c and loopback control sections 45a, 45b, and 45c. The ATM apparatuses 41a, 41b, and 41c are performing a loopback test by transmitting a test loopback cell to each other.
FIG. 6 shows the format of a conventional ATM cell. An ATM cell 51 is constituted by a 5-byte ATM header and a 48-byte payload which is an information field. This ATM header has a 12-bit VPI (Virtual Path Identifier), a 16-bit VCI (Virtual Channel Identifier), a 3-bit PT (Payload Type), a 1-bit CLP (Cell Loss Priority), and a 1-byte HEC (Header Error Control).
The ATM header is used to interface between ATM switching networks. Some ATM header is used to interface with a user network. In such an ATM header, a 4-bit GFD (Generic Flow Control) is added to 1-byte data defined in the ATM apparatuses 41a, 41b, and 41c to form a 12-bit VPI.
In the system having the above arrangement, the control section 45a of the ATM apparatus 41a generates one loopback cell, indicates that this cell is a test loopback cell, and enters a test value determined by the manager of the ATM switching network in the cell. In addition, after the ATM apparatus 41c is set as a turning point, and the ATM apparatus 41a is set as a start point, CRC (Cyclic Redundancy Check) operation is performed.
Subsequently, the designation of a VPI and a VCI which are to be tested is added to this test loopback cell, and the resultant cell is inserted in an idle time between the respective user cells and transmitted through the forward path 42a. In addition, the time managing section 44a is notified of the transmission of the loopback cell. The time managing section 44a then start measuring the elapsed time while holding the current time. In the ATM apparatus 41b, the loopback control section 45b receives a loopback cell from the ATM apparatus 41a, recognizes that the ATM apparatus 41b itself is a passing point, and transfers the cell to the ATM apparatus 41c through the forward path 42b.
In the ATM apparatus 41c, the loopback control section 45c receives the loopback cell and recognizes that the ATM apparatus 41c itself is a turning point. The loopback control section 45c rewrites the loopback cell into a backward loopback cell, and returns it to the ATM apparatus 41b through the backward path 43b. In the ATM apparatus 41b, the loopback control section 45b recognizes that the ATM apparatus 41b itself is a passing point as in the case of reception through the forward path 42a, and transfers the backward loopback cell to the ATM apparatus 41a through the backward path 43a.
In the ATM apparatus 41a, the loopback control section 45a receives the backward loopback cell and recognizes that the ATM apparatus 41a itself is an end point. The loopback control section 45a then recognizes the test value and notifies the time managing section 44a of the backward loopback cell. The time managing section 44a stops measuring the elapsed time upon reception of this notification, and clears the measured value. If the backward loopback cell is not received by the ATM apparatus 41a within a predetermined reference time monitored by the time managing section 44a due to some trouble, the time managing section 44a notifies the loopback control section 45a of a timeout. As a result, a loopback failure is recognized.
Since the reference time monitored by the time managing section 44a is set to be sufficiently longer than the sum of delay times in the forward paths 42a and 42b and the backward paths 43a and 43b, no loopback cell causes a timeout due to a normal delay.
When each transmission path is to be tested by using such a conventional loopback cell control system, the following problems are posed.
First, since the elapsed time until the reception of a loopback cell is measured as the overall elapsed time through the entire route for a loopback cell test, a delay time in each of the forward paths 42a and 42b and the backward paths 43a and 43b cannot be evaluated.
Second, even if some abnormality occurs in the ATM apparatus 41b or 41c, and an abnormal delay occurs in only the forward paths 42a and 42b or the backward paths 43a and 43b, when the backward loopback cell is received within a reference time, it is determined that the system is normal because of the first problem.